This disclosure is related to the field of directional drilling wellbores through subsurface formations. More specifically, the disclosure relates to methods and systems for drilling such wellbores along a selected trajectory using “steerable” hydraulically powered drilling motors.
U.S. Pat. No. 7,810,584 issued to Haci et al. describes a method and system for automatically operating a drilling system using a “steerable” hydraulically powered drilling motor disposed within a drill pipe “string” in conjunction with rotation of the drill pipe string from the surface. Rotation from the surface may be performed using, for example, a top drive or a kelly/rotary table. The drilling motor may have a housing with a slight bend in its shape, such that when the drilling motor alone is used to rotate a drill bit at the lower end portion of the drill pipe string, and the drilling motor is held in a selected rotational orientation, the trajectory of the wellbore tends to move in a direction of the interior of the bend in the housing. When the entire drill pipe string is rotated, the wellbore trajectory tends to continue in a substantially straight line. Thus, during directional drilling operations, a system operator may change or maintain the wellbore trajectory by stopping drill pipe string rotation, orienting the drilling motor in a selected direction and continuing drilling by using just the drilling motor to rotate the drill bit.
Systems and methods disclosed in the Haci et al. '584 patent may be used to increase drilling efficiency during such periods of time when the drill pipe string is not rotated (called “slide drilling”). In the most general terms, such systems and methods automatically rotate the drill string back and forth between selected surface-measured torque values, such that axial friction between the drill pipe string and the wall of the wellbore is reduced, while not causing substantial change in the orientation (called “toolface angle” or simply “toolface”) of the drilling motor.
The systems and method described in the Haci et al. '584 patent, as well as U.S. Pat. Nos. 7,096,979, 6,918,453 and 6,802,378, have been shown to provide improvement in drilling efficiency when a wellbore is drilled such that there is substantial lateral displacement of the well trajectory from its surface location (i.e., the starting point of the well).
Many wellbores drilled to have such lateral displacement may also have a portion thereof which is substantially vertical. At a selected depth in the wellbore, directional drilling may be initiated by stopping rotation of the drill pipe string such that the drilling motor is oriented in a selected direction and commencing slide drilling. During such initial part of directional drilling, there is relatively low friction between the wellbore wall and the drill pipe string. Under such conditions, the toolface orientation may be maintained by applying a torque to the drill pipe string at the surface using a rotary table or top drive as described above. Such surface applied torque is needed to offset reactive torque generated by the drilling motor when the drill pipe string is allowed to move into the wellbore so that the drill bit at the end portion thereof drills the subsurface formations.